Tunable High-Power High-Brightness Vertical-External-Cavity Surface-Emitting Lasers and Their Applications

Abstract

The extraction of high power with high beam quality from semiconductor lasers has long been a goal of semiconductor laser research. Optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) have already shown the potential for their high power high brightness operation. In addition, the macroscopic nature of the external cavity in these lasers makes intracavity nonlinear frequency conversion quite convenient. High-power high-brightness VECSELs with wavelength flexibility enlarge their applica-tions. The drawbacks of the VECSELs are their poor spectral characteristics, thermal-induced wavelength shift and a few-nm-wide linewidth.The objective of this dissertation is to investigate tunable high-power high-brightness VECSELs with spectral and polarization control. The low gain and microcavity reson-ance of the VECSEL are the major challenges for developing tunable high-power VECSELs with large tunability. To overcome these challenges, the V-shaped cavity, where the anti-reflection coated VECSEL chip serves as a folding mirror, and an extremely low-loss (at tuned wavelength) intracavity birefringent filter at Brewster's angle are employed to achieved the high gain, low-loss wavelength selectivity and the elimination of microcavity. This cavity results in multi-watt TEM00 VECSELs with a wavelength tuning range of 20~30 nm about 975 nm. Also the longitudinal mode discrimination introduced by birefringent filter makes the linewidth narrow down to 0.5 nm. After the tunable linearly polarized fundamental beam is achieved, the tunable blue-green VECSELs are demonstrated by using type I intracavity second-harmonic generation. The spectral control of VECSELs makes it possible to apply them as an efficient pump source for Er/Yb codoped single-mode fiber laser and to realize the spectral beam combining for multi-wavelength high- brightness power scaling.In this dissertation, theory, design, fabrication and characterization are presented. Rigorous microscopic many-body theory of the quantum well gain, based on semiconductor Bloch equations and k.p theory, is introduced. The closed loop design tool based on this theory is not only used to design the VECSEL structure, but also used as a precise on-wafer diagnostics tool by the experiment/theory comparison of the photo-luminescence. The characterization of the wafer shows that the modeling is in good agreement with the measured results.The VECSEL high power high brightness performance relies on the fabrication of the chip. The fabrication method of the VECSEL chip, which provides the optically smooth surface and good heat dissipation, is presented. The anti-reflection coating on the chip surface can significantly improve the slope efficiency of VECSEL when high reflectivity output coupler is used. Over 12-W VECSEL cw output power with 43 % slope efficiency is demonstrated at 0 oC. A beam quality factor (M^2 factor) of 1.75 is obtained at 11 W output power.